The present invention relates to the manufacture of alkali metal hydroxides, commonly known as caustic. Alkali metal hydroxides are typically from the group of elements which includes sodium, lithium, cesium, potassium and rubidium. Caustic is made using different types of electrolytic cells, including membrane and diaphragm-type cells and then further treating the caustic to concentrate the solution. The present invention is directed to a method of further treating sodium hydroxide caustic which was created using a diaphragm-type electrolytic chlorine cell, but it is contemplated that this method of manufacture can be utilized in other alkali metal hydroxide processes as well.
A need has long existed in the caustic industry for a technique to remove nickel and or iron from a caustic manufacturing process which requires the additional use of sodium borohydride without adding still more chemicals to the manufacturing process.
A need has existed for an environmentally friendly metal removal process, particularly to remove iron and nickel which lowers the cost to manufacture at least two grades of caustic, standard and bleach grade.
Alkali metal hydroxide solutions are manufactured by the electrolysis of alkali metal salt solutions in electrolytic cells. Diaphragm cells are known for producing alkali metal hydroxide solutions electrolytically. In this type of cell, a porous cathode with an overlying porous diaphragm is used to separate or to serve as a barrier between the catholyte and anolyte compartments of the cell. After separation of the components, the caustic solution is then further treated to increase the concentration of caustic in the solution.
An objectionable feature of this subsequent treatment of the caustic is that the step of adding sodium borohydride has been required to control the amount of chemical reaction between the caustic and the first device for driving water out of the aqueous caustic solution. This device is commonly known as the "first effect evaporator." The first effect evaporator is usually nickel or coated nickel, such as nickel 200, and the caustic of the solution normally reacts with the nickel causing the build up of hydrated nickel oxide on the processing equipment. Traditionally, the sodium borohydride has reduced this chemical reaction.
The process of manufacture of caustic has been expensive because of the need for large amounts of sodium borohydride, a costly substance only available from two sources. There has been a need for a process for treating a caustic solution saturated with nickel and iron compounds, as well as free metal ions, nickel and iron oxides, and particulate amounts of nickel and iron, without the need for the large quantities of sodium borohydride.
The present invention is directed at a method for reducing the amount of free nickel and nickel oxides, free iron and iron oxides in the chlor-alkali systems, particularly for the diaphragm types of cells, without the need for large quantities of sodium borohydride to complete the manufacturing process.
In order to overcome the disadvantages of the prior art the present invention proposes the use of magnets in situ in the manufacturing process to overcome these problems caused by large quantity sodium borohydride addition.